When turning a computer on and off, various peripherals (such as printers, screens, scanners, etc.) often have to be turned on and off as well. By using the 5-V supply voltage from the USB interface on the PC, all these peripherals can easily be switched on and off at the same time as the PC. This principle can also be used with other appliances that have a USB interface (such as modern TVs and radios). This so-called ‘USB standby-killer’ can be realized with just 5 components. The USB output voltage provides for the activation of the triac opto-driver (MOC3043) which has zero-crossing detection.
This, in turn, drives the TRIAC, type BT126 The circuit shown is used by the author for switching loads with a total power of about 150 W and is protected with a 1-A fuse. The circuit can easily handle much larger loads, however. In that case and/or when using a very inductive load a so-called snubber network is required across the triac. The value of the fuse will also need to be changed as appropriate. The circuit can easily be built into a mains multi-way powerboard. Make sure you have good isolation between the USB and mains sections (refer to the Electrical Safety page published regularly in this magazine).
The MOC303XM and MOC304XM devices consist of a GaAs infrared emitting diode optically coupled to a monolithic silicon detector performing the function of a zero voltage crossing bilateral triac driver.
They are designed for use with a triac in the interface of logic systems to equipment powered from 115 VAC lines, such as teletypewriters, CRTs, solid−state relays, industrial controls, printers, motors, solenoids, and consumer appliances, etc.
- Simplifies Logic Control of 115 Vac Power
- Zero Voltage Crossing
- DV/DT of 2000 V/µs Typical, 1000 V/µs Guaranteed
- To order devices that are tested and marked per VDE 0884 requirements, the suffix ”V” must be included at end of the part number. VDE 0884 is a test option. Recommended for 115/240 Vac(RMS) Applications:
- Solenoid/Valve Controls • Temperature Controls
- Lighting Controls • E.M. Contactors
- Static Power Switches • AC Motor Starters
- AC Motor Drives • Solid State Relays
